Electrical time delay line



Dec. 11, 1945. a L TAVQNEY 2,390,563

ELECTRICAL TIME DELAY LINE Filed Oct. 14, 1941 PHASE OUTPUT SHIFT I 4| INVENTOR |L-- UT OFF L. v 42 1 FREQUENCY 1 WWW M ATTORNEY Patented Dec. 11, 1945 ELECTRICAL TIME DELAY LINE Gereld L. Tawney. Hempstead, N. Y., minor to Sperry Gyroscope Company, N. Y., a corporation of New York Inc., Brooklyn,

Application Ootobel' 14,1941,Serial N 414,928

2 Claims.

This invention relates, generally, to the use of multi-section uniform filter networks as time delay lines, and, more specifically, to a novel type of delay line designed to give linear and calculable phase shift as a function of frequency and nearly constant amplitude of output with frequency.

It is well known in network theory and practice that a constant K type of low pass multisection uniform filter made up of T sections produces a phase characteristic which is approximately linear with frequency on the low frequency side of the band passed by the filter. This quantity K for such a filter is a constant at all frequencies, and is defined as the square root of the product of the series and shunt impedances function a linear one over a considerably longer part of the pass band. This is indicated in the article Extensions to the theory and design of electric wave filters by O. J. ZobehBell System Technical Journal, April 1931, pp. 284-341 (Mon- I ograph 3-556) a For instance, let us define a new constant ko by the formula:

where Mo is the mutual inductance between adjacent series inductances L of the network. The

, constant k may be referred to as the coupling coefiicient for the section. It has been found empirically that the phase characteristic can be made more linear if k0 is made to lie in the range .25 ko .45 and much more so for the lesser range .35 ko .40, when suitable mutual inductance between alternate coils is provided as'discussed below.

Such multi -section filters are suitable for use as delay networks for steady state frequencies or for transients, so long as the frequency components of such input signals lie on the linear portion of the phase characteristic, i. e. somewhat below the cut-off frequency. In delay networks, it is also necessary that this cut-off frequency be sufiiciently high to pass all or most of the components of the input signal with little relative attenuation. If these two requisites are satisfied, the output signal ,is free from phase and amplitude distortion, and an input transent signal will emerge from the network unchanged in form, though reduced in amplitude.

One of the main difllculties in the practical realization of such a design lies in the physical positioning of the series inductive elements of the filter so that interaction between them will be proper, i. e., so that kc will have a useful value. Another difilculty lies in the necessity for mamtaining high Q inductive elements so that the network gives small attenuation. The value of Q (which is defined for a coil in the art by the formula:

where w is the frequency, L the inductance of the coil, and R1. its alternating current resistance) is frequency dependent for most types of coils, and

so the coil design shouldmake Rx. approximately a direct function of frequency. Shielding from,

external efiects is, of course, necessary at all frequencies at which such delay networks are :most useful.

I have found that the coupling k1, between any pair of alternate coils defined by serves as a second order correction to the phase characteristic. The optimum value of In for values of ko such that .35 ko .40 is found to be of the order of .02 to .06.

It is then the object of this invention to prm vide a uniform multi-section T element delay" network of novel design and construction which has a linear phase characteristic up to very hi'gh portion cf the radio frequencies and over a large pass band.

Another object of thi invention is the provimanufacture and modification, and said design being readily shielded from external electromagnetic, electrostatic, thermal, vibratoryand other disturbances.

of the network (kl being Still another object of this invention is to provide such a delay network having properties making it possible to use portions of the delay time available in the network and allowing the network to be used in cascade with other similar networks to provide a greater delay time than is obtainable in one unit of such a delay net-' work.

Yet another object of this invention is the provision of an inductance whose Q is approximately frequency-independent and. one adapted for use in groups in delay or other networks, wherein the mutual inductance between adjacent coils and between alternate coils is chosen to still further improve the phase frequency characteristics of the delay line.

Other objects and advantages will become apparent from the specification, taken in connec-Y tion with the accompanying drawing wherein one embodiment of the invention is illustrated.

. In the drawing,

Fig. l is a partial cross section of the delay network, showing the compact distribution of its component elements.

Fig. 2 is an equivalent circuit of Fig. l. v Fig. 3 is-a cross section view taken along the line 33 of Fig. 1 and turned 90 degrees.

Fig. 4 is a perspective view of one of the coil cores as used in Fig. 1.

Fig. 5 shows a cross section of a series of these inductances as used in a network. Fig. 6 is a modification of Fig. 5.

Litz or solid wire, dependingon the desired cutoif frequency. The coil is then slipped into the annular groove l8, its lead wires being brought outside the core through notches 23 and 24. For

' 'screw 28 is adjusted against plate 2| which may Fig. 7, is a graph useful in explaining the characteristics of the delay network.

Other objects and advantages of this invention will become apparent as the description proceeds.

The physical arrangement of the filter network of my invention is shown in Fig. 1, Fig. 2 being an equivalent circuit for the network of Fig.

,The network is mounted inside of case l of suitable length which, as. shown in Fig. 3, may be of rectangular cross-section. The network is made up of a seriesof T elements, as typified by inductance 8 and 9 and capacitance I5 of Figs. land 2. I

The novel design of the inductances 3, 5, 6,

. 1, 8, 9 being one of the chief features of my centages by altering the geometry of the material of the coil, or by altering the design of the coil used in the core.- Thecore is shaped as shown in Fig. 4 in perspective and in Fig. 5 in cross section. An annular groove 18 formed by be of Bakelite or other material until the coils are firmly held in place.

Condensers 10, ll, I2, l3, l4, It: may be of any compact low loss type, such as the silver-mica rectangular molded condensers well known to the radio art. Their .si'des being around fiat. one lead of each condenser is soldered to ground wire 30, the other lead of each condenser being projected through the suitable holes in insulating holder 29, as seen' in Fig. 8, and soldered to the coillead wires, as shown at 3| and 32 in Fig. 1. Terminal condensers I0 and I6 are made half'the value of the condensers H, l2, i3, M,

I 5, so that delay lines of equal characteristic impedance may be cascaded together without the use of any external matching sections.

Grommets 33 and 34 are provided in end plates 2 and 3 for shielded terminal wires 35 and 36,

respectively. Adjustability in time delay may i be had by providing leads which tap the network at any desired time delay, such as shielded leads 3! and 38, which illustrate the manner in which the time delay may be decreased by either once or twice the delay introduced by each T-section. These taps may be placed line, however.

Holes 39 and 40, normally plugged when only one unit of the network is used, may be used for the lead wires when it is desirable to stack such units one on top of the other to obtain multiples of the delay time obtainable from a single unit.

As illustrated in Fig. 1, the use of either of these.taps 31, 38 would require that s very high load impedance be used with the tap selected,

and that the unused end of the line have placed across it a terminating impedance (across leads 30 and 35). It is obvious that such impedances. together with any necessary switching arrangements, might also be placed in the delay line case. Fractional delay times may be also easily provided by inserting Bakelite or other spacers in place of a number of T-sections removed.

All of the remaining emptyv space inside the filter case may be filled with any of the well known types of wax or pitch used for such purposes in radio manufacture, thus safe-guarding the network from the destructive effects of humidity,

the die in the material provides a recess for the coil, the outer wall I! providing proper shielding from external disturbances and slight coupling to non-adjacent inductances, the reentrant portion l9 providing a centralizing holder and magnetic circuit closer for the coil, and the back wall 20 providing a proper value of the coupling Ico to the adjacent inductance in the series. The

coils themselves are made up of a suitable number of turns of, preferably, honeycomb-wound vibration, et cetera.

To adjust the phase and amplitude character- I istics of the inductances. as shown by graphs 42 and ll, respectively, of Fig. 7, so that the phase is linear and the output fiat over as'large a frequency range as possible, the values of kc and in may usually be varied by altering the dimensions of. the wall I! or base 20, orby many other changes in dimension, separately, or in combination, or by changing the properties of the core material or of the coil. It has been found that the adjustments in Ice and In which give a linear v at any point in the line, including its characteristic impedance Zk and its delay time T, as defined by the formulae:

an open-ended shielding core and a coil positioned therein, means for holding said cores in a and coil of 3.6 millihenries has a delay time of 120 microseconds with attenuation of about 12 decibels and a characteristic impedance of 7000 ohms over a range Whose upper limit is very near a cut-oil frequency of 550 kilocycles. Thus, a

signal with no frequency components above 500 kilocycles will passthrough the network unchanged in shape, reduced in power by 12 decibels, and delayed 120 microseconds in time. It has beenfound that this type of delay line has useful properties up to a cut-off frequency of 25 megacycles.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. An electrical delay line embodying a substantially uniform network comprising a plurality of series-connected inductances each comprising row, whereby the open end of one core is closed by the closed end of the succeeding core, the wall thicknesses of said cores being chosen to provide a coupling coeificient between adjacent coils of a value between .35 and .40, and a coupling coefficient between alternate coils of the i order of .02 to .06, and a plurality of capacitative shunt elements substantially uniformly dis 'tributed along said line.

2. An electrical delay line embodying a substantially uniform network comprising. a plurality of series-connected inductors each comprising an open-ended shielding core having a. tubular outer wall, a disc end section, a reentrant portion, and a coil positioned on said reentrant portion within said tubular outer wall; means for holding said cores in a row in uniform alignment with said reentrant portions extending from the respective disc end sections in a. predetermined direction along a common axis of said series-connected inductors; the dimensions of said disc end sections being so chosen in relation to the dimensions of said reentrant portions and said outer tubular walls as to provide a desired coupling coefiicient between .25 and .40 between adjacent inductors, and a desired coupling coefllcient between .02 and .06 between alternate inductors; and a capacitor connected between a common conductor and each electrical circuit junction of adjacent inductors, whereby substantially uniform efiective time delay is provided in said delay line over an extremely wide frequency range.

GERELD L. TAWNEY. 

